The Bamenda Volcano (BV) (2621 m) is a stratovolcano situated in the Cameroon Line (CL). BV includes Mount Lefo (2534 m) which is situated on its southern slopes and contains one elliptical caldera (3 × 4 km). This caldera is propitious to farming and breeding activity. Despite these profitable assets, Lefo caldera (LC) is an amphitheater of the occurrence of multi-origin hazards that have direct or indirect impacts on the biodiversity and human patrimony. The most present hazards are those of meteorological origin. Numerous combined factors (steepest slopes, heavy rainfalls, weathered state of volcanic products…) rule these hazards. These factors gave rise to the occurrence of landslides, gullies erosion and rock falls which occur precisely on caldera northern and eastern rims. Hazards of anthropogenic origin are based on the destruction of the vegetation cover by the population for dealing, firewood and building issues. Moreover, during the breeding activity, the cattle covers the caldera throughout the day; this unevens the topography and destroys the meadow. Hazards of volcanological origin are not yet occurring in LC; but the recent Lake Monoun (1984) and Nyos (1986) CO 2 eruptions, mount Cameroon eruptions (1999 and 2000) and mount Oku Lake event (2011) which are close to the BV and, the age of 0 Myr of basalt, constitutes a subject of controversy toward the reactivation of the Cameroonian hotspot faults. The assessment of risks in LC was based on the average income of breeding activity and house investment . The economy of LC is valued at about US$527472.527. The level of such risk will be important in case of loss of human beings and the destruction of geomorphosites’ values. In order to reduce the level of looses in this zone, hazard and risks maps are provided in this paper as well as some recommendations.
The Cameroon country is crossed by a particular geological structure called Cameroon Line (CL) (
Volcanic apparatus of the CL present relevant economical assets; therefore, they are liable to the settlement of active agropastoral multi-ethnic populations although the occurrence of numerous disastrous hazards. Accordingly, some studies focused on hazards and related risks have been carried out in some CL’s volcanoes in order to mitigate and reduce the risks rate in case of hazards [
on biodiversity and human patrimony.
Geological Context of the Study AreaLC appears as the smallest caldera of BV an is elliptical in shape (3 × 4 km) opened to the south-east. It was developed by successive emplacement from 0 to 27, 4 Myrs on the south-eastern slopes of BV precisely between 10˚11' - 10˚15'E and 05˚46' - 05˚50'N. Its basement consists of Pan-African granitoids [
Numerous hazards are found in LC; they are classified according to the type of their triggering factors. The most common hazards in the region are those of meteorological, anthropogenic and volcanological origin.
These hazards include mass movements (rock falls, debris falls and landslides) and the effect of high velocity of wind on the goods. Mass movements in LC are triggered by the combination of predisposing and triggering factors.
The predisposing factors of mass movements include: lithology, structural setting, climate, slope morphology, vegetation cover and land use.
· Lithology
The lithology of LC (
· Structural setting
The structural setting of LC is highlighted by fractures and geological contacts. The hydrographic network
(
It is indicative of the basement fractures system induced by the large-scale Precambrian fault reactivation [
· Climate
Since LC is a mountainous region, the climate conditions are aggressive and they are Sudanese Tropical type [
· Slope morphology
The slope morphology in LC is heterogeneous.
Method of the realization of slopes morphology map: As in a whole set of GIS software, slope maps used in this study have been calculated from a Digital Elevation Model (DEM) with a 30 × 30 m spatial resolution using ILWIS (Integrated Land and Water Investigation Software). The DEM was provided by the online database of the U.S. Geological Survey: http://gdex.cr.usgs.gov/gdex/.
The calculation consists of three steps. Firstly, two gradient filters are calculated and saved as layers (Dx and Dy). i.e. the filter which detects slope differences in x-direction (DFDX), and the filter which detects slope differences in y-direction (DFDY). Secondly, the slope map is calculated by typing the following formula in the Map calculation command line: Slope (degrees) = RADDEG (ATAN ((HYP(Dx, Dy))/pixsize (Dem))).
In this formula HYP (hypotenuse) is an internal map calculation function to determine the positive root of the sum of square Dx plus square Dy (Pythagoras rule). The ATAN function calculates the arctan (tan−1), and returns real values in radians in the range −1/2π to 1/2π. The function RADDEG is used to convert the radians to degrees.
Finally, the slope map is sliced by grouping the slope values into a set of classes according to the different ranges which better illustrate the five classes of slopes that can influence or not mass movement hazards.
The wavy class appear much distributed in the caldera floor and in the valleys close to the caldera. The steep class appears on the caldera rims and on escarpments related to the caldera system.
· Vegetation cover
The vegetation of LC (
· Land use
LC has natural trumps that attract important active population including Bamenda, Bakossi and Bororo people who are estimated at around 60 persons. Leading activities are farming and breeding. They are practiced throughout the year. Land use system is poor. Agriculture is led on the caldera’s floor precisely on flat areas (
breeders stay on the crest line of the caldera.
In many cases triggering factors of mass movements can be at short term and middle term. In LC, triggering factors include extreme meteorological events and human activities.
· Extreme meteorological events
Extreme meteorological events include long-lasting rainfalls that occur precisely in June, July, August and September (
· Human activities
Human activities that can trigger mass movements in LC are the opening of road and pedestran tracks, road traffic, excavation and use of engines during farming activities.
Meteorological crises and human activities (farming and breeding) led to the deposition of boundless mellow and crumbly cover with low instability at the basement (presence of sliding zones). This situation favors the occurrence of phenomena such as rocks falls (
velocity [
Moreover, basement fissures induced the formation of deeper (2 - 3 m) and longer (15 - 50 m) gullies scattered in the northern slopes of LC (
The vegetation cover is threatened during human activities; trees are cut by some families for dealing, firewood and building issues and for farms enlargement (
Moreover, during the breeding activity, the cattle covers the caldera throughout the day; this destroys the meadow and uneven the topography (
These hazards are absent in the LC but the recent phenomena recorded in some CL’s volcanoes: recent Lake
Monoun and Nyos CO2 eruptions (respectively in 1984 and 1986), the mount Cameroon eruptions in 1999 and 2000 as well as that of mount Oku Lake in 2011 close to BV (
The biodiversity, human beings and human patrimony in LC will be affected by lava flows and pyroclastic explosions (
The assessment of risk in the LC is based on the economical income from breeding activity and house investments (
LC constitutes a geosite made up of several geomophological units (domes, escarpments and valleys) called geomorphosites. The rarity, the uniqueness, the history and the dynamic of such geomorphosites gave rise to student training, geotourism and many recreational manifestations in the region. Geotourism is one of the best activities which can improve the economical situation of LC’inhabitants; therefore it must be developed. LC geomorphosites are characterized by ecological value due to the fact that they are the support of flora and fauna species in the region.
Activities | Average units | Average unit costs (FCFA) | Total costs (FCFA) | |
---|---|---|---|---|
Breeding | Beefs | 390 | 300,000 | 136,500,000 |
Sheeps | 100 | 35,000 | 3,500,000 | |
Investments | HOUSES | 50 | 2,000,000 | 100,000,000 |
Total | 240,000,000 FCFA ≈US$527472.527 |
LC is threatened by three main types of hazards. The hazards of meteorological origin are the most widespread in the region. Landslide and rock falls are most representative. They are ruled by the combination of several predisposing and triggering factors. The climatic conditions are very aggressive on the geological features. The level of rain is very high during rainy months and wind current velocity is significant. Waters penetrate the weakness zones of rocks (spaces between columns and fissures) and create multiple cracks that are in some cases induced by vegetation components roots. Moreover, water also percolates into the ground and is responsible of soil overloading and deep-seated weathering. The vegetation components are mainly meadow (grasses) and small colonies of eucalyptus; accordingly root reinforcement is lacking. This is probably because tree roots do not penetrate below the soil cover. The increasing impact of human activities, such as intensive agriculture, quarrying, road construction, urbanization, land use changes, etc., is also responsible for slope instability and landslide hazard [
Hazards of volcanological origin are not occurring in LC. However, in view of recent phenomena recorded in some CL’s volcanoes close to BV and, structural and geochronological data, the volcanic activities can resume at anytime. The lithology of the region is indicative of type of eruption. The outcropping of basalts, Basantes, trachytes and ignimbrites testify that eruptions in LC could be mixed and will include fissural, extrusive and explosive types.
The negative effects of landslides are generally more pronounced in developing countries because of their high “social vulnerability” to natural disasters, due to economical, social, political and cultural constraints [
LC is an amphitheater tormented by numerous natural and man-made hazards. Hazards of meteorological origin (rock falls, debris falls and landslides) are the most frequent and well distributed hazards. They are ruled by the combination of natural and human factors. Hazards of anthropogenic origin are attributed to the unconsciousness of the population. Hazards of volcanological origin are not found actually in LC but some indices testify the resumption of volcanic activities at anytime. The economical assets of the region gave rise to the settlement of active population that leads the cost-effective breeding and farming activities. This situation increases the level of risk; indeed, the recommendations become compulsory:
· Populations must be sensitized about natural hazards that they are facing and on climate change. Indeed, documents highlighting relevant example of such hazards in some threatened localities around the world must be provided to the population. Risks map produced in this paper can be used as a tool to draw the attention of LC inhabitants. Some panels most be installed in LC, indicating the prohibited area for constructions and activities. Moreover, experts on climate change must explain to the population the negative effect of deforestation and bushfires on the climate.
· Avoid deforestation, bushfires and implement reforestation.
· Amalgamate the stock breeders in a single location and create stock farms to avoid over pasturing.
· Politics must take into account hazards and risks maps during the management of the LC.
This work was validated after slight discussions with the late Professor Wandji Pierre who has deceased in April 2014; our posthumously thanks are addressed to him. The authors thank the Executive committee of the International Association of Geomorphologists for their support to attending the training course on landslide mapping in Dessie (Ethiopia) that helps them to improve the brand of this work. They also express their gratitude right side the traditional authorities for their hospitality during the field works.